The incident solar radiation at the surface of the earth has fairly low intensity, somewhat variable depending on the time of the day, season and location of the earth; roughly 800-1000 watts of thermal energy per square meter of area. For the solar industry to become commercially competitive in diverse global markets as compared to the costs of energy production from fossil industry (such as coal-generated energy), the key challenges for the solar industry are to harness the solar radiation at high efficiency and significantly low costs, and improve both the ease of global manufacturability of solar technologies and the ease of assembly and implementation by the conventional commercial sector for diverse applications.
There are primarily four categories of solar energy harnessing technologies. The first of which, denoted as passive solar devices, take the form of solar heaters and roof top solar panels with pipes running underneath that are simply heated from incident solar radiation; the heat transferring to the fluid or air passing through the passive solar devices. However, passive solar radiation intensity is generally too low to be directly useful for most applications that require heat at high temperatures and pressures (such as running HVAC, industrial process heat, turbines, etc.). Another category of solar technology takes of the form of flat panels of solar photovoltaic cells that contain semiconductor devices. These devices convert incident solar radiation directly into electricity; the electrical energy output is a direct function of the area of the PV cell array. Solar “concentrators” are considered as another category and are designed to reflect the incident solar radiation from a large area onto a smaller focal area in order to concentrate (collect) the solar radiation to generate large amounts of heat that can be transferred to a fluid circulating at the focal area (thus achieving higher temperature and pressure), suitable for various applications where heat of desired quality is needed, such as industrial process heat, HVAC heating and cooling, desalination, dehydration etc., or for steam production to drive turbines in order to generate electricity. Recently, a hybrid technology has been explored, where solar collectors are used in conjunction with a special type of PV cell; the solar concentration technology directs a high intensity solar radiation beam onto a specialized small number of photovoltaic cells (called concentrator PV cells) at the focal area of the collectors. Thus, concentrating solar collectors in conjunction with PV cells have an advantage over flat-panel collectors in that they utilize substantially smaller amounts of semiconductor material, while also being more efficient in generating electrical energy.
In any event, solar concentrator technologies are most widely useful whether the application needs heat or electricity or both. As compared to passive solar technologies or PV cell technologies, solar concentrators provide heat at low-costs and high efficiency.
One particular type of concentrating collector is known in the art as a parabolic trough collector. This type of collector uses an elongated reflective trough having a parabolic cross-section to concentrate the sun's radiation along a focal line extending through the focal points of the parabolic elements forming the trough. A conduit is typically positioned along this focal line, with a heat-transferring liquid circulating through the conduit. The liquid will be heated by the sun's energy as it moves through the conduit.
Alternatively, a solar concentrator may take the form of a parabolic point concentrator, using a plurality of curved mirrors arranged to form a parabolic dish that focuses the incoming solar radiation onto a single point. A Stirling engine or cavity receiver is typically placed at the focal point of the dish concentrator to capture the solar energy and convert it into heat (or mechanical) energy.
Yet another variation of solar concentrator technology is referred to as a “power tower” or “heliostat”, where hundreds of individual parabolic point reflectors are arranged in a solar field and each reflector focuses its beam onto the top of a large central tower where the receiver is housed. The advantage of this configuration is that the heat collection from a large collection area of the field is centralized to minimize heat loss and cost of piping. However, the shadowing effects, large land requirements, and other considerations are some of the disadvantages of the power tower types of concentrator technologies.
Recently, a hybrid concentrator technology has been developed called a Concentrated Linear Fresnel Reflector (CLFR) where there is a hybrid of trough shape reflectors are arranged in a field with a central linear receiver. That is, instead of a small receiver area on the tower, the CLL uses a long receiving pipe that collects focal beams of an array of parabolic reflectors.
One problem with these and other types of conventional concentrating solar collectors is that they are expensive to fabricate and install. Additionally, the configurations are usually “application specific” in terms of being designed for the specific geographic conditions of a given site (i.e., a function of the local movements of the sun), or requiring highly planar field for fine mirror alignment, or requiring a large tract of land in a desert, etc.). Each implementation also has requirements for specific dimensions (in terms of their surface area, lengths, etc.), offering little flexibility to adapt to application-specific situation. Moreover, the solar collector optics are generally comprised of glass reflectors that must be painstakingly constructed and polished, require costly supporting structures, and thereafter require extensive and tedious installation.
Thus, a need remains in the art for an improved solar concentrator with efficient operation, while being relatively simple to install and maintain, and ubiquitous in how they are used (to produce heat, electricity, or both) and as few or as many as needed.